189 research outputs found
Graviton propagation within the context of the D-material universe
Motivated by the recent breakthrough of the detection of Gravitational Waves
(GW) from coalescent black holes by the aLIGO interferometers, we study the
propagation of GW in the {\sl D-material universe}, which we have recently
shown to be compatible with large-scale structure and inflationary
phenomenology. The medium of D-particles induces an effective mass for the
graviton, as a consequence of the formation of recoil-velocity field
condensates due to the underlying Born-Infeld dynamics. There is a competing
effect, due to a super-luminal refractive index, as a result of the
gravitational energy of D-particles acting as a dark matter component, with
which propagating gravitons interact. We examine conditions for the condensate
under which the latter effect is sub-leading. We argue that if quantum
fluctuations of the recoil velocity are relatively strong, which can happen in
the current era of the universe, then the condensate, and hence the induced
mass of the graviton, can be several orders of magnitude larger than the
magnitude of the cosmological constant today. Hence, we constrain the graviton
mass using aLIGO and pulsar timing observations (which give the most stringent
bounds at present). In such a sub-luminal graviton case, there is also a
gravitational Cherenkov effect for ordinary high energy cosmic matter, which is
further constrained by means of ultra-high-energy cosmic ray observations.
Assuming cosmic rays of extragalactic origin, the bounds on the quantum
condensate strength, based on the gravitational Cherenkov effect, are of the
same order as those from aLIGO measurements, in contrast to the case where a
galactic origin of the cosmic rays is assumed, in which case the corresponding
bounds are much weaker.Comment: 21 pages, JCAP style, no figure
The D-material universe
In a previous publication by some of the authors (N.E.M., M.S. and M.F.Y.),
we have argued that the "D-material universe", that is a model of a brane world
propagating in a higher-dimensional bulk populated by collections of D-particle
stringy defects, provides a model for the growth of large-scale structure in
the universe via the vector field in its spectrum. The latter corresponds to
D-particle recoil velocity excitations as a result of the interactions of the
defects with stringy matter and radiation on the brane world. In this article,
we first elaborate further on the results of the previous study on the galactic
growth era and analyse the circumstances under which the D-particle recoil
velocity fluid may "mimic" dark matter in galaxies. A lensing phenomenology is
also presented for some samples of galaxies, which previously were known to
provide tension for modified gravity (TeVeS) models. The current model is found
in agreement with these lensing data. Then we discuss a cosmic evolution for
the D-material universe by analysing the conditions under which the late eras
of this universe associated with large-scale structure are connected to early
epochs, where inflation takes place. It is shown that inflation is induced by
dense populations of D-particles in the early universe, with the role of the
inflaton field played by the condensate of the D-particle recoil-velocity
fields under their interaction with relativistic stringy matter, only for
sufficiently large brane tensions and low string mass scales compared to the
Hubble scale. On the other hand, for large string scales, where the
recoil-velocity condensate fields are weak, inflation cannot be driven by the
D-particle defects alone. In such cases inflation may be driven by dilaton (or
other moduli) fields in the underlying string theory.Comment: 42 pages latex, one pdf figure incorporated, uses special macro
Plasma 5-hydroxyindoleacetic acid as an endogenous index of renal plasma flow
Plasma 5-hydroxyindoleacetic acid as an endogenous index of renal plasma flow. 5-Hydroxyindoleacetic acid (5-HIAA), the major serotonin metabolite, was measured in human plasma and urine using liquid chromatography and electrochemical detection. The renal extraction of 5-HIAA was 78%, a value compatible with an almost complete extraction during a single passage of the blood through the kidney. In addition, plasma levels of 5-HIAA measured in patients with a wide range of renal function was inversely correlated (r = 0.85) with the clearance of para-aminohippuric acid (PAH). The results indicate that 5-HIAA determinations can be used as an alternative procedure to the PAH clearance method for the estimation of renal plasma flow in clinical practice
Chronic ethanol attenuates centrally-mediated hypotension elicited via alpha-2-adrenergic, but not I1-imidazoline, receptor activation in female rats
Aims—This study dealt with the effect of chronic ethanol administration on hemodynamic responses
elicited by α2-adrenergic (α-methyldopa) or I1-imidazoline (rilmenidine) receptor activation in
telemetered female rats.
Main methods—The effects of α-methyldopa or rilmenidine on blood pressure (BP), heart rate
(HR) and their variability were investigated in rats that received liquid diet without or with ethanol
(5% w/v) for 12 weeks. To evaluate the effect of each drug on cardiovascular autonomic control (BP
and HR variability) in the absence or presence of ethanol, three time-domain indices of hemodynamic
variability were measured: (i) standard deviation of mean arterial pressure (SDMAP), (ii) standard
deviation of beat-to-beat intervals, and (iii) root mean square of successive differences in R-R
intervals.
Key findings—In liquid diet-fed control rats, i.p. rilmenidine (600 μg/kg) or α-methyldopa (100
mg/kg) reduced BP along with decreases and increases, respectively, in HR. Both drugs had no effect
on HR variability but reduced BP variability (SDMAP), suggesting a reduced vasomotor sympathetic
tone. Ethanol feeding attenuated reductions in BP and SDMAP evoked by α-methyldopa but not by
rilmenidine.
Significance—We conclude that chronic ethanol preferentially compromises α2- but not I1-
receptor-mediated hypotension in female rats probably via modulation of vasomotor sympathetic
activity. These findings highlight the adequacy of rilmenidine use to lower BP in hypertensive
alcoholic females
Sympathetic Activation and Baroreflex Function during Intradialytic Hypertensive Episodes
BACKGROUND: The mechanisms of intradialytic increases in blood pressure are not well defined. The present study was undertaken to assess the role of autonomic nervous system activation during intradialytic hypertensive episodes. METHODOLOGY/PRINCIPAL FINDINGS: Continuous interbeat intervals (IBI) and systolic blood pressure (SBP) were monitored during hemodialysis in 108 chronic patients. Intradialytic hypertensive episodes defined as a period of at least 10 mmHg increase in SBP between the beginning and the end of a dialysis session or hypertension resistant to ultrafiltration occurring during or immediately after the dialysis procedure, were detected in 62 out of 113 hemodialysis sessions. SBP variability, IBI variability and baroreceptor sensitivity (BRS) in the low (LF) and high (HF) frequency ranges were assessed using the complex demodulation technique (CDM). Intradialytic hypertensive episodes were associated with an increased (n = 45) or decreased (n = 17) heart rate. The maximal blood pressure was similar in both groups. In patients with increased heart rate the increase in blood pressure was associated with marked increases in SBP and IBI variability, with suppressed BRS indices and enhanced sympatho-vagal balance. In contrast, in those with decreased heart rate, there were no significant changes in the above parameters. End-of-dialysis blood pressure in all sessions associated with hypertensive episode was significantly higher than in those without such episodes. In logistic regression analysis, predialysis BRS in the low frequency range was found to be the main predictor of intradialytic hypertension. CONCLUSION/SIGNIFICANCE: Our data point to sympathetic overactivity with feed-forward blood pressure enhancement as an important mechanism of intradialytic hypertension in a significant proportion of patients. The triggers of increased sympathetic activity during hemodialysis remain to be determined. Intradialytic hypertensive episodes are associated with higher end-of-dialysis blood pressure, suggesting that intradialytic hypertension may play a role in generation of interdialytic hypertension
Cusps and pseudocusps in strings with Y-junctions
Contains fulltext :
135362.pdf (preprint version ) (Open Access
Gravitational wave bursts from cosmic string cusps and pseudocusps
We study the relative contribution of cusps and pseudocusps, on cosmic
(super)strings, to the emitted bursts of gravitational waves. The gravitational
wave emission in the vicinity of highly relativistic points on the string
follows, for a high enough frequency, a logarithmic decrease. The slope has
been analytically found to be for points reaching exactly the speed
of light in the limit . We investigate the variations of this high
frequency behaviour with respect to the velocity of the points considered, for
strings formed through a numerical simulation, and we then compute numerically
the gravitational waves emitted. We find that for string points moving with
velocities as far as from the theoretical (relativistic) limit ,
gravitational wave emission follows a behaviour consistent with that of cusps,
effectively increasing the number of cusps on a string. Indeed, depending on
the velocity threshold chosen for such behaviour, we show the emitting part of
the string worldsheet is enhanced by a factor with respect to
the emission of cusps only.Comment: 26 pages, 18 figures; Added referenc
- …